The quantity-metering accuracy of injectors, in particular servo-controlled piezo injectors, is subject to tolerances in production which make injector-specific correction (classification) of the individual injectors necessary. This classification of the individual injectors is used in the injection system (engine/vehicle) to correct the deviations from one injector to another. Such early classification of an injector with respect to its quantity tolerance, for example during final function testing during production, can, however, not ensure that no quantity-related change in the injection of the injector has taken place up to the first time it is put into service in the system. In other words, after the final function testing, quantity-related changes may still occur which are not taken into account in the preceding classification.
On the other hand, determination of the absolute injection quantities of the injectors during the operation of the system is linked to defined operating states of the system, for example a sufficiently long thrust phase at a constant operating temperature. Such quantity measurement and resulting calculation of the quantity deviation with respect to QSETP with subsequent correction can therefore under certain circumstances not be carried out precisely and reliably for a long time. This results in that quantity deviations owing to production tolerances may already be present beforehand during operation, with the result that uncorrected injection processes have already occurred.
Such quantity-related changes of the injector which occur early are due, in particular, to an idle-stroke-dependent component of the injector. Such an idle-stroke-dependent component usually exhibits high dynamics. As a result, significant changes in the idle stroke of the injector may be exhibited from the production and functional classification of the injector up to the time when it is first put into service in the system (vehicle/engine), (rapid idle stroke drift).
The term “idle-stroke-dependent” can be explained as follows. The tolerances which are responsible for the quantity variations in injectors can be divided into two groups. The first group is dependent on the idle stroke of the drive (piezo-drive) of the injector, while the second group is formed by a component which is independent of the drive. The tolerance component which is independent of the idle stroke usually has low dynamics (for example seat wear at the needle seat or at the servo valve seat). Corresponding correction strategies in the system therefore have a sufficiently long time to detect and to correct a change in the tolerances which are independent of the idle stroke. In contrast, the idle-stroke-dependent component usually exhibits high dynamics. A correction strategy which is intended to counteract this therefore has to be capable of detecting a change in the idle stroke from the first operation of the injector and to correct it in the system. However, solely determining the current idle stroke of the injector in the system is not sufficient. In order to correct an idle-stroke-based change in quantity of an injector, the current idle stroke should be known during the quantity classification of the injector.
The following summary can therefore be made: solely classifying the injector with respect to its quantity tolerance during final function testing during production is therefore not sufficient since up to the time of putting into service quantity-related changes may have taken place which are due, in particular, to idle-stroke-dependent tolerance components. On the other hand, quantity adjustment which is carried out during the operating phase for correction purposes is dependent on specific operating phases and therefore, under certain circumstances, cannot take place until relatively late, with the result that in the preceding operating phase uncorrected injections have already taken place.
In German patent application 10 2010 021 168.0, which was not published before the priority date of the present document, methods are described for detecting the idle stroke of the actuator of an injector in a hydraulic and/or electrical way. These methods may be carried out continuously and when a change in idle stroke is detected a corresponding correction of the injection time of the injector can be carried out.
DE 102 57 686 A1 discloses a method for adapting the characteristic of an injection valve of an internal combustion engine to age-related changes in an actual injection behavior, wherein rotation speed values of the internal combustion engine for work cycles of the injection valve are detected with and without actuation, a difference between the detected values is formed, and therefore a correction of the injection characteristic is performed.